An encoding maps a Unicode character to an encoded sequence of bytes. A particular encoding is represented by a type that is derived from the Encoding class. Specifically, a character is encoded to a byte sequence by calling the encoding type's Encoding.GetBytes(String) method, and the byte sequence is decoded to a character array or a string by calling the Encoding.GetChars(Byte[]) or Encoding.GetString(Byte[]) method.

An encoding operation can fail if the input character cannot be represented by the encoding. For example, an ASCIIEncoding object cannot encode a character whose Unicode code point value is outside the range U+0000 to U+007F.

When an encoding conversion cannot be performed, the .NET Framework provides a failure-handling mechanism called a fallback. Your application can use predefined .NET Framework encoder fallbacks, or it can create a custom encoder fallback derived from the EncoderFallback and EncoderFallbackBuffer classes.

EncoderFallback and EncoderFallbackBuffer are the base classes for all encoding fallback handlers in the .NET Framework. They support the following three kinds of fallback handling mechanisms:

Best-fit fallback, which maps valid Unicode characters that cannot be encoded to an approximate equivalent. For example, a best-fit fallback handler for the ASCIIEncoding class might map Æ (U+00C6) to AE (U+0041 + U+0045). A best-fit fallback handler might also be implemented to transliterate one alphabet (such as Cyrillic) to another (such as Latin or Roman). The .NET Framework does not provide any public best-fit fallback implementations.

Replacement fallback, which replaces each character that cannot be encoded with a predefined string. The .NET Framework provides a predefined replacement fallback handler. The EncoderReplacementFallback class replaces each byte sequence that cannot be decoded with a question mark character ("?", or U+003F) or a REPLACEMENT CHARACTER (U+FFFD). You can customize the replacement string by specifying a substitute in the call to the EncoderReplacementFallback.EncoderReplacementFallback(String) constructor. After the substitute string is emitted, the encoding operation continues converting the remainder of the input.

Exception fallback, which throws an exception when a character cannot be encoded. The .NET Framework provides a predefined exception fallback handler. The EncoderExceptionFallback class throws an EncoderFallbackException when an invalid character is encountered, and the encoding operation terminates.

If you choose to implement a custom solution, you must override the following abstract members of the EncoderFallback class: